LAUSR

We analyze space‐time variations in the depth distribution of seismicity in Southern and Baja California, focusing on transients following four M ≥ 6.7 mainshocks. The regular brittle‐ductile transition depth is estimated at different locations as the local bottom of 99,636 background events and is compared with the bottom of events within earthquake clusters. The four M ≥ 6.7 mainshock‐aftershock sequences exhibit early aftershocks with depths up to 5 km below the regular brittle‐ductile transition depth and epicentral distances up to 15 km from the mainshock ruptures. The maximum aftershock depth increases abruptly following the mainshocks and recovers to the background level after several years. The wide‐spread deeper‐than‐usual early aftershocks favor classical brittle‐ductile transition over change from unstable to stable frictional response as the mechanism governing the base of the seismogenic zone. Episodic transient deepening of the brittle‐ductile transition following major earthquakes can have important long‐term effects on the lower crust.